Whats the objective? Pressurize the tank? Fit a cap like this if you dont mind the judges hemming and hawwing about it. Remember to open the vent or your only gonna make it a block before you start scratching yer head... troop

That's not really practical, and the item in the Lang's link will not do what you're suggesting. As John suggests, the pressure build-up in the gas tank, when the outside temperature rises, will cause your carburetor to leak.

If everything else is sealed well, your carb doesn't drip, your tank doesn't leak or weep fuel, and all your fuel connections are dry, the fumes emitting from the gas cap vent should be unnoticeable.